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Not all VoIP gateways need to support SS7. There are many different types of VoIP gateways that suit different applications. For example, there are many analog (FXS) VoIP gateways on the market that simply allow an analog device such as a phone or fax machine to communicate over an IP network. As another example, there are VoIP gateways that support T1 PRI ISDN. These could be common in enterprises deploying VoIP internally but using the PSTN to connect calls externally.
SS7 stands for Signaling System number 7. It is a messaging protocol used in telephony networks worldwide which handles call control and provides many other services. SS7 typically does not operate at the network edge, rather, it is used for communicating between switches and other network elements inside the telephony network. To learn more about SS7, there is a good tutorial a href=http://www.iec.org/online/tutorials/ss7/> here.
Basically a VoIP gateway only needs to support SS7 for applications where it is desired to connect from the IP network directly into the SS7 network. Service providers that are partnered with other phone companies and Internet Telephony Service Providers ITSPs would likely use such a gateway.
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